Solar cell

ABSTRACT

A solar cell comprises a substrate configured to have a plurality of via holes and a first conductive type, an emitter layer placed in the substrate and configured to have a second conductive type opposite to the first conductive type, a plurality of first electrodes electrically coupled to the emitter layer, a plurality of current collectors electrically coupled to the first electrodes through the plurality of via holes, and a plurality of second electrodes electrically coupled to the substrate. The plurality of via holes comprises at least two via holes having different angles.

This application is a Continuation of application Ser. No. 12/559,551filed on Sep. 15, 2009 now U.S. Pat. No. 7,838,762, which claimspriority to and the benefit of Korean Patent Application No.10-2009-0042220 filed in the Korean Intellectual Property Office on May14, 2009. The entire contents of all of the above applications arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solar cell.

2. Discussion of the Related Art

In recent years, with the expected exhaustion of the existing energyresources such as petroleum or coal, there is a growing interest inalternate energy sources which will substitute for existing energyresources. From among them, a solar cell is configured to generateelectrical energy from solar energy, and it has been in the spotlightbecause solar energy can be easily obtained and has no problems withenvironmental pollution.

A typical solar cell includes a substrate and an emitter layer made ofsemiconductors having different conductive types such as a p type or ann type, and electrodes coupled to the substrate and the emitter layer.Here, a PN junction is formed at the interface of the substrate and theemitter layer.

When light is incident on the solar cell, a plurality of electron-holepairs are generated from the semiconductors, and the generatedelectron-hole pairs are separated into electrons and holes (i.e.,electric charges) by a photovoltaic effect. The electrons and the holesmove toward the n-type semiconductor and the p-type semiconductor (e.g.,the emitter layer and the substrate) and are collected by the electrodeselectrically coupled to the substrate and the emitter layer. Theelectrodes are interconnected by electric wires, thereby allowingelectric power to be obtained.

Here, one or more current collectors, such as bus bars coupled to theelectrodes coupled to the emitter layer and the substrate, are placedover the emitter layer and the substrate. The current collectorsfunction to move electric charges, collected by correspondingelectrodes, to an external load through adjacent current collectors.

In this case, the current collectors are placed over not only thesubstrate on which light is not incident, but the emitter layer formedon a surface on which light is incident (i.e., a light-receivingsurface). Accordingly, the efficiency of the solar cell is low becausean area on which light is incident on the substrate is decreased by thecurrent collectors.

In order to reduce the decrease in the efficiency of the solar cellresulting from placement of the current collectors on a light-receivingsurface of the substrate, a metal wrap through (MWT) solar cell, etc.,have been developed wherein the current collectors coupled to theemitter layer through via holes are placed on the rear surface of thesubstrate, placed on an opposite side to the light-receiving surface.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a solar cell mayinclude a substrate configured to have a plurality of via holes and afirst conductive type; an emitter layer placed in the substrate andconfigured to have a second conductive type opposite to the firstconductive type; a plurality of first electrodes electrically coupled tothe emitter layer; a plurality of current collectors electricallycoupled to the first electrodes through the plurality of via holes; anda plurality of second electrodes electrically coupled to the substrate,wherein the plurality of via holes comprise at least two via holeshaving different angles.

The at least two angles may be different depending on places where thevia holes are formed.

The at least two via holes may have different horizontal sectionalshapes.

As the angle increases, the horizontal sectional shape of the via holemay be close to a circle.

The at least two via holes may have different sectional areas.

A size of the sectional area may increase as the angle of the via holedecreases.

The plurality of via holes may include at least two via holes having asame angle.

The at least two via holes may be placed in a same row.

The at least two via holes may be placed in a same column.

The at least two via holes may be placed at a same distance from viaholes having a maximum angle.

Each of the different angles may be about 45° to 90° with the substrate.

According to an aspect of the present invention, a method of forming asolar cell includes forming a plurality of via holes in a substrate of afirst conductive type; forming an emitter layer in the substrate, theemitter layer being a second conductive type opposite to the firstconductive type; forming a plurality of first electrodes electricallycoupled to the emitter layer; forming a plurality of current collectorselectrically coupled to the first electrodes through the plurality ofvia holes; and forming a plurality of second electrodes electricallycoupled to the substrate, wherein the plurality of via holes are formedso that at least two via holes have different angles.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, in which:

FIG. 1 is a partial perspective view of a solar cell according to anembodiment of the present invention;

FIG. 2 is a sectional view of the solar cell taken along line II-II inFIG. 1;

FIG. 3 is a diagram showing via holes formed in a substrate according toan embodiment of the present invention;

FIG. 4 include diagrams showing examples of a position where a laserbeam is irradiated, and examples of angles and horizontal sectionalshapes of via holes according to a direction where the substrate movesin accordance with an embodiment of the present invention;

FIG. 5 include diagrams showing examples of angles and horizontalsectional shapes of via holes according to an initial position of a viahole formation apparatus in accordance with an embodiment of the presentinvention;

FIG. 6 is a diagram showing angles and horizontal sectional shapes ofvia holes according to the position of the substrate when the pluralityof via holes are formed using two via hole formation apparatusesaccording to an embodiment of the present invention; and

FIG. 7 include diagrams showing changes in the magnitude of a currentcollector for a front electrode existing within a via hole according toan angles of the via hole in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings in order for thoseskilled in the art to be able to implement the invention. As thoseskilled in the art would realize, the following embodiments may bemodified in various different ways and the present invention is notlimited to the embodiments. Wherever possible, the same referencenumbers will be used throughout the drawings to refer to the same orlike parts.

To clarify multiple layers and regions, the thickness of the layers isenlarged in the drawings. Like reference numerals designate likeelements throughout the specification. When it is said that any part,such as a layer, film, region, or plate, is positioned on another part,it refers to the part being directly on the other part or above theother part with at least one intermediate part. On the other hand, ifany part is said to be positioned directly on another part it refers tothere being no intermediate part between the two parts. In thespecification, when it is said that any part is “entirely” formed onanother part, it refers to the part being not formed on not only theentire surface of the other part, but part of the edge of the surface.

A solar cell according to an embodiment of the present invention isdescribed below with reference to the accompanying drawings. First, asolar cell according to an embodiment of the present invention isdescribed in detail with reference to FIGS. 1 to 7.

FIG. 1 is a partial perspective view of a solar cell according to anembodiment of the present invention. FIG. 2 is a sectional view of thesolar cell taken along line II-II in FIG. 1. FIG. 3 is a diagram showingvia holes formed in a substrate according to an embodiment of thepresent invention. FIG. 4 includes diagrams showing examples of aposition where a laser beam is irradiated, and examples of angles andhorizontal sectional shapes of via holes according to a direction wherethe substrate moves in accordance with an embodiment of the presentinvention. a to e of FIG. 5 are diagrams showing examples of angles andhorizontal sectional shapes of via holes according to an initialposition of a via hole formation apparatus in accordance with anembodiment of the present invention. FIG. 6 is a diagram showing anglesand horizontal sectional shapes of via holes according to the positionof the substrate when the plurality of via holes are formed using twovia hole formation apparatuses according to an embodiment of the presentinvention. FIG. 7 includes diagrams showing changes in the magnitude ofa current collector for a front electrode existing within a via holeaccording to an angle of the via hole in accordance with an embodimentof the present invention.

Referring to FIG. 1, the solar cell 1 according to the embodiment of thepresent invention includes a substrate 110 having a plurality of viaholes 181 formed therein, an emitter layer 120 placed in the substrate110, an anti-reflective layer 130 placed on the emitter layer 120 on asurface (hereinafter referred to as a ‘front surface’) of the substrate110 on which light is incident, a plurality of first electrodes 140(hereinafter referred to as ‘front electrodes’) placed on the emitterlayer 120 on the front surface of the substrate 110 where theanti-reflective layer 130 is not placed, second electrode (hereinafterreferred to as ‘rear electrode’) 150 placed on a surface (hereinafterreferred to as a ‘rear surface’) of the substrate 110 on which light isnot incident and which is placed to face the front surface, a pluralityof current collectors 161 (hereinafter, referred to as ‘front electrodecurrent collectors’) for the front electrodes 140, a plurality ofcurrent collectors 162 (hereinafter, referred to as ‘rear electrodecurrent collectors’) for the rear electrode 150 electrically connectedto the rear electrode 150, and a back surface field (BSF) unit (orlayer) 170 interposed between the rear electrode 150 and the substrate110. Here, the front electrode current collectors 161 are isolated fromthe rear electrode 150, are each placed in the via holes 181 and theemitter layer 120 adjacent to the via holes 181, and are coupled to thefront electrodes 140.

The substrate 110 is a semiconductor substrate made of silicon having afirst conductive type (e.g., a p conductive type). Here, the silicon maybe single crystalline silicon, polycrystalline silicon, or amorphoussilicon. In the case where the substrate 110 has a p conductive type,the substrate 110 may contain an impurity of a group III element, suchas boron (B), gallium (Ga), or indium (In). However, the substrate 110may be an n conductive type and may be made of semiconductor materialsother than silicon. In the case where the substrate 110 has an nconductive type, the substrate 110 may contain an impurity of a group Velement, such as phosphorous (P), arsenic (As), or antimony (Sb).

The substrate 110 has the plurality of via holes 181 penetratingtherethrough and has a surface that is textured, thus having a texturedsurface (i.e., an uneven surface). The plurality of via holes 181 areformed in the substrate 110, and, more particularly, at respectiveportions where the plurality of front electrodes 140 and the frontelectrode current collectors 161 cross each other.

In the present embodiment, a longitudinal sectional shape of each of thevia holes 181 when the via hole 181 is cut in a direction perpendicularto the central axis of the via hole 181 is circular, but is not limitedthereto. For example, the longitudinal sectional shape of the via hole181 may have a variety of shapes, such as an oval or a polygon.

The via hole 181 may have a different angle with the surface of thesubstrate 110 according to a position where the via hole 181 is formed.That is, the plurality of via holes 181 includes at least one via hole181, formed in a direction substantially perpendicular to the surface ofthe substrate 110, and a plurality of via holes 181 inclined withrespect to the surface of the substrate 110. Accordingly, there exist atleast two via holes 181 with different angles, but there also exist thevia holes 181 having the same angle according to their positions. Here,a tilt angle and a tilt direction of the via hole 181 for the surface ofthe substrate 110 vary depending on a position where the via hole 181 isformed in the substrate 110.

Further, a horizontal sectional shape of the via hole 181 when the viahole 181 is cut in parallel to the surface of the substrate 110 differsdepending on an angle of the via hole 181. Thus, the sectional area ofthe via hole 181 also differs depending on the angle of the via hole181. Accordingly, the via holes 181 having the same angle have asubstantially similar horizontal sectional shape. In the presentembodiment, since the longitudinal sectional shape of the via hole 181is circular, the horizontal sectional shape of the via hole 181 is closeto a circle with the angle of the via hole 181 becoming greater.Accordingly, the sectional area of the via hole 181 decreases. On thecontrary, the horizontal sectional shape of the via hole 181 has an ovalhaving a gradually wider width with the angle of the via hole 181becoming smaller. Accordingly, the sectional area of the via hole 181increases.

In the present embodiment, the angle of the via hole 181 ranges fromabout 45° to 90°. The difference in the angle between the via hole 181having a maximum angle and the via hole 181 having a minimum angle iswithin a range of about 45°. In the present embodiment, the angle refersto, as described above, an angle formed by the via hole 181 and thesurface of the substrate 110. The tilt angle refers to an acute angle ofthe via hole 181 inclined with respect to the surface of the substrate110.

In the present embodiment, the tilt angle of the via hole 181 for thesubstrate 110 may be a tilt angle for a surface of the substrate 110 onwhich texturing has not been performed, but is not limited thereto.

The emitter layer 120 is an impurity portion having a second conductivetype (e.g., an n conductive type) opposite to the conductive type of thesubstrate 110 and is configured to form a PN junction together with thesemiconductor substrate 110.

Electrons-hole pairs (i.e., electric charges) generated by lightincident on the substrate 110 are separated into electrons and holes bya built-in potential difference resulting from the PN junction.Consequently, the electrons move toward the n type and the holes movetoward the p type. Accordingly, if the substrate 110 has a p conductivetype and the emitter layer 120 have an n conductive type, the separatedholes move toward the substrate 110, and the separated electrons movetoward the emitter layer 120. Accordingly, the holes become the majoritycarriers in the substrate 110, and the electrons become the majoritycarriers in the emitter layer 120.

The emitter layer 120 forms the PN junction together with the substrate110. Accordingly, in the case where the substrate 110 has an nconductive type unlike the present embodiment, the emitter layer 120 hasa p conductive type. In this case, the separated electrons move towardthe substrate 110, and the separated holes move toward the emitter layer120.

In the case where the emitter layer 120 has an n conductive type, theemitter layer 120 may be formed by doping the substrate 110 with animpurity of a group V element, such as phosphorous (P), arsenic (As), orantimony (Sb). On the contrary, in the case where the emitter layer 120has a p conductive type, the emitter layer 120 may be formed by dopingthe substrate 110 with an impurity of a group III element, such as boron(B), gallium (Ga), or indium (In).

The anti-reflective layer 130, made of silicon nitride (SiNx), siliconoxide (SiOx), etc., is formed on the emitter layer 120 on the frontsurface of the substrate 110. The anti-reflective layer 130 functions toreduce reflectance of light incident on the solar cell 1 and increasethe selectivity of a specific wavelength region (or band) of incidentlight, thereby improving the efficiency of the solar cell 1. Theanti-reflective layer 130 may have a thickness ranging from about 70 nmto about 80 nm. In some embodiments, the anti-reflective layer 130 mayalso be placed on sidewalls of the via holes 181. The anti-reflectivelayer 130 may be omitted, if necessary.

Although not shown in FIG. 1, for performing edge isolation of thesubstrate 110, the anti-reflective layer 130 and the underlying emitterlayer 120 have exposure portions (not shown) exposing portions of thecorners of the front surface of the substrate 110.

The plurality of front electrodes 140 are placed on the emitter layer120 formed on the front surface of the substrate 110, and areelectrically coupled to the emitter layer 120. The plurality of frontelectrodes 140 extend in a predetermined direction with the frontelectrode 140 being isolated from each other, and cover the via holes181 placed beneath.

The front electrodes 140 collect electric charges (e.g., electrons) thatmove toward the emitter layer 120 and transfer the electric charges tothe front electrode current collectors 161 electrically coupled theretothrough the via holes 181. The front electrodes 140 are made of at leastone conductive material. The conductive material may include, forexample, at least one selected from the group consisting of nickel (Ni),copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn), indium(In), titanium (Ti), gold (Au), and a combination thereof, but may bemade of other conductive material.

The front electrode current collectors 161 are placed on the rearsurface of the substrate 110. As shown in FIGS. 1 and 2, the frontelectrode current collectors 161, also referred to as bus bars, extendin a direction intersecting the front electrodes 140 placed on the frontsurface of the substrate 110.

The front electrode current collectors 161 are also made of at least oneconductive material. The front electrode current collectors 161 areconfigured to intersect the front electrodes 140 and coupled to thefront electrodes 140 through the via holes 181. Since the frontelectrode current collectors 161 are electrically coupled to the frontelectrodes 140, the front electrode current collectors 161 outputelectric charges, which are received from the front electrodes 140 to anexternal device.

The conductive material of the front electrode current collectors 161may include, for example, at least one selected from the groupconsisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin(Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and acombination thereof, but may be made of other conductive material. Inthe present embodiment, although the front electrode current collectors161 are illustrated to include the same material as the front electrodes140, the front electrode current collectors 161 may include differentmaterials from the front electrodes 140.

The rear electrode 150 is placed on the rear surface of the substrate110 and electrically coupled to the substrate 110. The rear electrode150 is isolated from adjacent front electrode current collectors 161.The rear electrode 150 functions to collect electric charges (e.g.,holes) moving toward the substrate 110.

The rear electrode 150 is made of at least one conductive material. Theconductive material may include, for example, at least one selected fromthe group consisting of nickel (Ni), copper (Cu), silver (Ag), aluminum(Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and acombination thereof, but may be made of other conductive material.

A plurality of exposure portions 182 is formed between the rearelectrode 150 and the respective front electrode current collectors 161.Thus, the rear electrode 150, the front electrode current collectors161, and the underlying emitter layer 120 include the plurality ofexposure portions 182 exposing parts of the rear surface of thesubstrate 110. The plurality of exposure portions 182 are chiefly formednear the front electrode current collectors 161. Accordingly, electricalconnections between the front electrode current collectors 161 formoving and collecting electrons or holes and the rear electrode 150 forcollecting holes or electrons are disconnected by each of the exposureportions 182, thereby making smooth the movement of the electrons andthe holes.

The rear electrode current collectors 162 are disposed in such a way asto electrically connect to the rear electrode 150. The rear electrodecurrent collectors 162 are made of a conductive material. The conductivematerial may include, for example, at least one selected from the groupconsisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin(Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and acombination thereof, but may be made of other conductive material.

The rear electrode current collectors 162 are disposed at regularintervals. Each of the rear electrode current collectors 162 includes aplurality of circular pads, but may include pads having various shapes,such as an oval or a square. Alternatively, the rear electrode currentcollector 162 may have a stripe shape extending in parallel to the frontelectrode current collectors 161.

The rear electrode current collectors 162 output electric charges (e.g.,holes) received from the rear electrode 150 electrically coupledthereto, to an external device.

In the present embodiment, the number of each of the front electrodecurrent collectors 161 and the rear electrode current collectors 162 maybe 2 or more, but may vary, if necessary.

The BSF unit 170 is placed between the rear electrode 150 and thesubstrate 110. The BSF unit 170 is a region (e.g., an n+ region) withwhich an impurity having the same conductive type as the substrate 110is doped at a concentration higher than that of the substrate 110.

A potential barrier is formed resulting from the difference in theimpurity concentration between the substrate 110 and the BSF unit 170,thus hindering the migration of electrons toward the rear surface of thesubstrate 110. Accordingly, electrons and holes can be reduced orprevented from becoming recombined through a recombination of theelectrons and the holes near the surface of the substrate 110.

In the solar cell 1 constructed as above, the plurality of via holes 181formed in the substrate 110 are formed at the portions of the substrate110 at which the front electrodes 140 intersect the front electrodecurrent collectors 161, as described above.

As described above, the via holes 181 form different angles togetherwith the surface of the substrate 110 depending on positions where thevia holes 181 are formed. Here, the magnitude of an angle θ and a tiltdirection change depending on positions where the via holes 181 areformed.

Assuming that, as shown in FIG. 3, the substrate 110 is divided into atotal of 9 virtual regions with a longitudinal width and a traversewidth being equally divided, an angle θ of each of the via holes 181formed in a virtual region A1 placed at the center of the substrate 110is about 90° with respect to the surface of the substrate 110. However,the angle θ of the via hole 181 gradually decreases with an increase inthe distance from the virtual region A1 (i.e., closer to the edges ofthe substrate 110). In the present embodiment, the angle θ of the viahole 181 may range from about 45° to 90°.

Since the angle θ of the via hole 181 is about 45° at minimum and about90° at maximum as described above, an average angle of each of thevirtual regions A1 to A9 ranges from about 45° to 90°. In the presentembodiment, the average angle of each of the virtual regions A1 to A9ranges from about 50° to 85°.

Further, the tilt directions of the via holes 181 for the substrate 110are oriented toward the virtual region A1 placed at the center of thesubstrate 110. That is, the tilt directions of the via holes 181 areoriented toward a via hole 181 having the greatest angle θ (i.e., a viahole 181 having the angle θ of about 90°) or a via hole 181 having anangle θ most close to about 90° when a via hole 181 having the angle θof about 90° does not exist, from among the via holes 181 formed in thevirtual region A1 at the center of the substrate 110.

When the plurality of via holes 181 is formed in the substrate 110 asshown in a to d of FIG. 4, the angle, horizontal sectional shape,sectional area, etc. of the via holes 181 formed in the substrate 110vary depending on a position relation between the substrate 110 and alaser beam irradiation apparatus 20, an irradiation (operation)direction of the substrate 110 or the laser beam irradiation apparatus20, etc. In the present embodiment, it is illustrated that an example ofa via hole formation apparatus is the laser beam irradiation apparatus20, but not limited thereto.

For example, the plurality of via holes 181 may be formed while movingboth the substrate 110 and the laser beam irradiation apparatus 20 asshown in a to d of FIG. 4.

In FIG. 4, the number of the front electrode current collectors 161 isthree and, as described above, the plurality of via holes 181 arechiefly formed at points where the front electrodes 140 intersect thefront electrode current collectors 161. Accordingly, the via holes 181are formed along each of the front electrode current collectors 161, asshown in FIG. 4. The number of the via holes 181 and the number of thefront electrode current collectors 161 may be changed. In someembodiments, the number of the front electrode current collectors 161may be two or four or more.

a and b of FIG. 4 show the via holes 181 formed in the substrate 110 inthe case where the substrate 110 moves in the direction of an arrow “A”or “B” (approximately an X-axis direction) and where the laser beamirradiation apparatus 20 irradiates a laser beam while moving up anddown. As shown in a and b of FIG. 4, the via holes 181 formed in thesame row have substantially the same angle, and the via holes 181 formedin different rows have different angles. The angles of the via holes 181decrease toward the lower side [a of FIG. 4] of the substrate 110 or theupper side [b if FIG. 4] of the substrate 110, and so the horizontalsectional area of each via hole 181 increases. That is, the horizontalsectional shape of the via hole 181 becomes close to a circle (at thistime, the angle is about 90°) toward the upper side [a of FIG. 4] or thelower side [b of FIG. 4] of the substrate 110.

c and d of FIG. 4 show the via holes 181 formed in the substrate 110 inthe case where the substrate 110 moves in the direction of an arrows “C”or “D” (approximately, a Y-axis direction) and where the laser beamirradiation apparatus 20 irradiates a laser beam while moving left andright. As shown in c and d of FIG. 4, the via holes 181 formed in thesame column have substantially the same angle, and the via holes 181formed in different columns have different angles. Accordingly, theangles of the via holes 181 decrease toward the left side of thesubstrate 110, and so the horizontal sectional shape of each via hole181 becomes close to a circle toward the right side of the substrate110.

In the case where, as described above, the plurality of via holes 181are formed while moving both the substrate 110 and the laser beamirradiation apparatus 20 in a predetermined direction, the via holes 181have varying angles according to their positions, and so the horizontalsectional shape and the sectional area of each via hole 181 alsochanges. Further, two or more via holes 181 having the same angle areformed according to a direction where the substrate 110 moves and adirection where the laser beam irradiation apparatus 20 irradiates alaser beam. In particular, there exist a row or a column consisting ofvia holes 181 having the same angle according to a shape where the viaholes 181 are arranged (e.g., in the case where the predetermined numberof via holes 181 is arranged in a matrix form in the row and columndirections of the substrate 110).

Consequently, the angle (and the horizontal sectional shape andsectional area) of each via hole 181 changes according to an irradiationdistance between the laser beam irradiation apparatus 20 and thesubstrate 110, varying depending on the position of the substrate 110.The angle of the via hole 181 decreases with the longer irradiationdistance, the via holes 181 formed by laser beams having differentirradiation distances have different angles (and different horizontalsectional shapes and sectional areas). The via holes 181 formed by laserbeams having the same irradiation distance have substantially the sameangle (and the same horizontal sectional shape and sectional area).

Moreover, the laser beam irradiation apparatus 20 may irradiate a laserbeam and the substrate 110 may move in various directions other than thedirections where the laser beam irradiation apparatus 20 irradiates alaser beam and the directions where the substrate 110 moves as shown ina to d of FIG. 4. Even in this case, the angle, horizontal sectionalshape, and sectional area of each via hole 181 change according to theirradiation distance of the laser beam.

In some embodiments, unlike the examples shown in FIG. 4, the pluralityof via holes 181 may be formed in the substrate 110 while changing onlythe position where the laser beam irradiation apparatus 20 irradiates alaser beam after fixing the substrate 110, as shown in FIG. 5.

a to e of FIG. 5 are diagrams showing examples of an angle andhorizontal sectional shape of the via hole according to an initialposition of the via hole formation apparatus in accordance with anembodiment of the present invention.

In order to form the via holes 181, the substrate 110 is placed at aninitial position in response to the laser beam irradiation apparatus 20,and the via holes 181 are formed at corresponding positions whilechanging a direction where the laser beam irradiation apparatus 20irradiates a laser beam. In the present embodiment, a position of thelaser beam irradiation apparatus 20, corresponding to the substrate 110when the substrate 110 moves to the initial position, is called theinitial position of the laser beam irradiation apparatus 20.Consequently, the laser beam irradiation apparatus 20 forms the viaholes 181 while changing the irradiation direction of the laser beam atthe initial position.

a of FIG. 5 shows an example in which the initial position of the laserbeam irradiation apparatus 20 is right at (or close to) the center ofthe substrate 110. b of FIG. 5 shows an example in which the initialposition of the laser beam irradiation apparatus 20 is at an upper leftcorner of the substrate 110. c of FIG. 5 shows an example in which theinitial position of the laser beam irradiation apparatus 20 is at anupper right corner of the substrate 110. Further, d of FIG. 5 shows anexample in which the initial position of the laser beam irradiationapparatus 20 is at a lower left corner of the substrate 110, and e FIG.5 shows an example in which the initial position of the laser beamirradiation apparatus 20 is at a lower right corner of the substrate110.

As shown in a to e of FIG. 5, the irradiation angle of a laser beambecomes small with an angle of the laser beam irradiation apparatus 20with respect to the substrate 110 (i.e., an irradiation angle of thelaser beam) becoming distant from the initial position having an angleof about 90° (i.e., with an irradiation distance of the laser beamirradiation apparatus 20 becoming longer). Accordingly, the via holes181 have smaller angles and wider sectional areas, as shown in a to e ofFIG. 5. That is to say, when the longitudinal sectional shape of the viahole 181 is a circle, the horizontal sectional shape of the via hole 181becomes close to a circle with the irradiation distance of the laserbeam irradiation apparatus 20 from the initial position becomingshorter, and so the sectional area of the via hole 181 decreases.

Here, the tilt direction of the via hole 181 radiates in the irradiationdirection of a laser beam as shown in a to e of FIG. 5. Accordingly, theplurality of tilted via holes 181 are oriented toward a via hole 181having the greatest angle. That is, the plurality of tilted via holes181 is oriented toward the initial position of the laser beamirradiation apparatus 20.

As described above with reference to FIG. 4, in FIG. 5, the horizontalsectional shape, sectional area, and angle of each of the via holes 181formed in the substrate 110 vary depending on the irradiation distanceof the laser beam, varying depending on the position of the substrate110. Accordingly, the via holes 181 formed at the same irradiationdistance from the initial position have the same angle, and the viaholes 181 formed at different irradiation distances from the initialposition have different angles. Consequently, the angle (and thehorizontal sectional shape and sectional area) of each of the via holes181 formed at the same distance from a via hole 181 having a greatestangle becomes the same.

FIGS. 4 and 5 show the examples in which the via holes 181 are formed atcorresponding portions of the substrate 110 using a single via holeformation apparatus 20. However, in some embodiments, the plurality ofvia holes 181 may be formed using two or more via hole formationapparatuses.

For example, as shown in FIG. 6, the plurality of via holes 181 may beformed using two via hole formation apparatuses 21 and 22. In this case,the via holes 181 formed by the different via hole formation apparatuses21 and 22 may have the same angle, and so may have the same horizontalsectional shape and sectional area. That is to say, the number of viaholes 181 each having an angle may be identical to at least the numberof via hole formation apparatus.

Further, from FIGS. 4 to 6, it can be seen that the inclined via holes181 are oriented toward a via hole 181 having the greatest angle becausethe tilt direction of the via hole 181 is related to the irradiationdirection of the laser beam.

The angle θ of the via hole 181, the amount of the front electrodecurrent collector 161 existing within the via hole 181, and a connectionrelation between the front electrode current collector 161 and the frontelectrode 140 depending on the amount of the front electrode currentcollector 161 are described below with reference to FIG. 7.

a of FIG. 7 is a diagram showing the amount of the front electrodecurrent collector existing within the via hole when the angle of the viahole is about 90°. b of FIG. 7 is a diagram showing the amount of thefront electrode current collector existing within the via hole when theangle of the via hole is about 45°. c of FIG. 7 is a diagram showing theamount of the front electrode current collector existing within the viahole when the angle of the via hole is about 30.

When the angle θ of the via hole 181 is about 90° as shown in a of FIG.7, the front electrode current collector 161 is completely filled withinthe via hole 181. That is, the conductive material for forming the frontelectrode current collector 161 is completely filled from a directionwhere the conductive material is introduced into the via hole 181 (e.g.,the bottom surface of the substrate 110 in FIG. 7) to the top surface ofthe substrate 110, and so the front electrode current collector 161 iscompletely filled within the via hole 181. Accordingly, the contactbetween the front electrode current collector 161 existing within thevia hole 181 and the front electrode 140 existing on the via hole 181 isfacilitated.

When the angle θ of the via hole 181 is about 45° as shown in b of FIG.7, the front electrode current collector 161 is filled within the viahole 181 only about half. That is, the conductive material for formingthe front electrode current collector 161 is filled from the bottomsurface of the substrate 110 to the top surface thereof, but theconductive material does not reach the entire top surface of the viahole 181 because of the tilt of the via hole 181 and fills only part ofthe top surface of the via hole 181. Accordingly, the front electrodecurrent collector 161 existing on part of the top surface of the viahole 181 comes into contact with the front electrode 140 existing on thevia hole 181.

Further, when the angle θ of the via hole 181 is about 30° smaller than45° as shown in c of FIG. 7, the front electrode current collector 161is filled up to a middle portion of the via hole 181. That is, theconductive material for forming the front electrode current collector161 is filled from the bottom surface of the substrate 110 to the topsurface thereof, but does not reach up to the top surface of the viahole 181 because the introduction of the conductive material is hinderedby the tilt surface of the via hole 181. Accordingly, since the frontelectrode current collector 161 existing in part of the via hole 181does not come into contact with the front electrode 140 existing on thevia hole 181, the front electrode 140 and the front electrode currentcollector 161 are not electrically connected to each other through thevia hole 181.

With the amount of the front electrode current collectors 161 (i.e., aconnection member existing within the via hole 181) increasing asdescribed above, the connection between layers through the via hole 181is facilitated.

In the present embodiment, the angle of the via hole 181 ranges fromabout 45° to about 90°. Accordingly, an electrical connection betweenthe front electrodes 140 (i.e., upper layers) and the front electrodecurrent collectors 161 (i.e., lower layers) is facilitated.Consequently, the failure rate of the solar cell 1 resulting from anincomplete connection between the front electrodes 140 and the frontelectrode current collectors 161 can be reduced.

The solar cell 1 constructed as above according to the presentembodiment is a metal wrap through (MWT) solar cell in which the frontelectrode current collectors 161 are placed on the rear surface of thesubstrate 110 on which light is not incident, and the front electrodecurrent collectors 161 and the front electrodes 140 placed on the frontsurface of the substrate 110 are connected to each other using theplurality of via holes 181. The operation of the solar cell 1 isdescribed below.

When light is irradiated to the solar cell 1 and incident on thesemiconductor substrate 110 through the anti-reflective layer 130 andthe emitter layer 120, electrons-hole pairs are created in thesemiconductor substrate 110 by light energy. Here, since the surface ofthe substrate 110 has experienced the texturing processing, thereflectance of light from the front surface of the substrate 110decreases, and the light is confined within the solar cell 1 through theincident and reflection operations on the textured surface. Accordingly,the absorption rate of light is increased, and so the efficiency of thesolar cell 1 is improved. Incidentally, the amount of light incident onthe substrate 110 is further increased because the reflection loss oflight incident on the substrate 110 is reduced by the anti-reflectivelayer 130.

The electrons-hole pairs are separated from each other by the PNjunction of the substrate 110 and the emitter layer 120. The separatedelectrons move toward the emitter layer 120 having an n conductive type,and the separated holes move toward the substrate 110 having a pconductive type. The electrons move toward the emitter layer 120 asdescribed above are collected by the front electrodes 140 and then movetoward the front electrode current collectors 161 electrically coupledto the front electrodes 140 through the via holes 181. The holes movetoward the substrate 110 are collected by corresponding rear electrodes150 through adjacent BSF unit 170 and then move toward the rearelectrode current collectors 162.

As described above, since the angle of each via hole 181 ranges fromabout 45° to about 90°, connections between the front electrodes 140 andthe front electrode current collectors 161 placed in the via holes 18 isreliable. Accordingly, the failure rate of the solar cell 1 isdecreased, and the operational efficiency of the solar cell 1 isimproved.

When the front electrode current collectors 161 and the rear electrodecurrent collectors 162 are interconnected by electric wires, current iscaused to flow and used externally as electric power.

According to a related method of forming a solar cell, the methodincludes forming a plurality of via holes in a substrate of a firstconductive type; forming an emitter layer in the substrate, the emitterlayer being a second conductive type opposite to the first conductivetype; forming a plurality of first electrodes electrically coupled tothe emitter layer; forming a plurality of current collectorselectrically coupled to the first electrodes through the plurality ofvia holes; and forming a plurality of second electrodes electricallycoupled to the substrate, wherein the plurality of via holes are formedso that at least two via holes have different angles.

According to the embodiments of the present invention, connectionsbetween the first electrodes and the front electrode current collectorsthrough the via holes is achieved safely. Accordingly, the failure rateof a solar cell can be decreased and the operational efficiency of thesolar cell can be improved.

While the present invention has been shown and described in connectionwith the example embodiments thereof, those skilled in the art willappreciate that the present invention may be changed and modified invarious ways without departing from the spirit and scope of the presentinvention as defined in the following claims.

1. A solar cell, comprising: a substrate configured to have a pluralityof via holes and a first conductive type; an emitter layer placed at afirst surface of the substrate and configured to have a secondconductive type opposite to the first conductive type; a plurality offirst electrodes on the emitter layer and coupled to the emitter layer;a plurality of first current collectors placed at a second surface ofthe substrate and into the plurality of via holes, and coupled to thefirst electrodes through the plurality of via holes, the second surfacebeing opposite to the first surface with respect to the substrate; aback surface field unit placed at the second surface of the substrate;and a plurality of second electrodes placed on the back surface fieldunit and coupled to the back surface field unit, wherein the pluralityof via holes comprise at least two via holes having different angles,and the at least two angles are determined by the position on thesubstrate where the at least two via holes are formed.
 2. A solar cellas claimed in claim 1, wherein the at least two via holes have differenthorizontal sectional shapes.
 3. A solar cell as claimed in claim 2,wherein as the angle of the via hole increases, the horizontal sectionalshape of the via hole becomes closer to a circle.
 4. A solar cell asclaimed in claim 1, wherein the at least two via holes have differentsectional areas.
 5. A solar cell as claimed in claim 4, wherein a sizeof the sectional area increases as the angle of the via hole decreases.6. A solar cell as claimed in claim 1, wherein each of the differentangles is about 45° to 90° with the substrate.
 7. A solar cell asclaimed in claim 1, further comprising a plurality of second currentcollectors placed on the second surface of the substrate and coupled tothe plurality of second electrodes.
 8. A solar cell as claimed in claim7, wherein the emitter layer is further placed at portions of the secondsurface of the substrate where the plurality of second currentcollectors are placed.
 9. A solar cell as claimed in claim 1, whereinthe first surface is an incident surface and the second surface is anon-incident surface.
 10. A solar cell as claimed in claim 1, furthercomprising an anti-reflective layer placed on the emitter layer.